1. Field of the Invention
The present invention relates in general to a straight travelling apparatus for construction vehicles such as excavators, cranes and, more particularly, to an apparatus for keeping the travelling direction of construction vehicles straight even when a work implement of the vehicle is being manipulated during travelling.
2. Description of the Prior Art
In typical construction equipment such as excavators, cranes and other construction equipment capable of travel, travelling actuators, i.e., travelling motors, and working actuators i.e., boom cylinders and arm cylinders for actuating booms and arms are operated by pressurized oil delivered from first and second hydraulic pumps of the same displacement capacity.
With reference to FIG. 1, there is shown a typical hydraulic circuit of a hydraulic excavator. As shown in this drawing, first and second hydraulic pumps P1 and P2 having the same capacity are driven by an engine E. The first pump P1 delivers the pressurized oil for operating a swing motor SW, a left travelling motor TL and an arm cylinder ARM, while the second pump P2 delivers the pressurized oil for operating a boom cylinder BM, a right travelling motor TR, an optional actuator OPT and a bucket cylinder BKT. In the drawing, the reference characters V1, V2 and V3 denote directional control valves for the swing motor SW, the left travelling motor TL and the arm cylinder ARM respectively, while the reference characters V4, V5, V6 and V7 denote direction control valves for the boom cylinder BM, the right travelling motor TR, the optional actuator OPT and the bucket cylinder BKT respectively.
The hydraulic circuit of FIG. 1 also includes a pair of center bypass lines 101 and 102 for directly returning the pressurized oil of the first and second pumps P1 and P2 to an oil tank T when all the directional control valves V1 to V7 are in their neutral positions.
The hydraulic circuit further includes a pair of parallel lines 103 and 104 for supplying exclusively or in parallel the pressurized oil of the pumps P1 and P2 to at least one actuator when at least one of the directional control valves V1 to V7 is being operated. The reference character Pi denotes a pilot pump for providing spool operating pressures for the valves V1 to V7, while the reference numerals 105a and 105b denote control vanes for the left and right travelling motors TL and TR respectively.
When the valves 105a and 105b for the left and right travelling motors TL and TR of the above hydraulic circuit are operated after starting the engine E, the pressurized oil of the first pump P1 is supplied to the left travelling motor TL through the first parallel line 103 and the second valve V2, while the pressurized oil of the second pump P2 is supplied to the right travelling motor TR through the second parallel line 104 and the fifth valve V5. At this time, the hydraulic excavator travels straightly on the work field since the capacities of the two pumps P1 and P2 are equal to each other.
When at least one work implement, the arm for example, is manipulated by supplying the pressurized oil of the first pump P1 to the arm cylinder ARM during the straight travelling of the vehicle, the pressurized oil of the pump P1 is simultaneously supplied to both the left travelling motor TL and the arm cylinder ARM through the first parallel line 103. Thus, the amount of pressurized oil supplied to the left travelling motor TL will be reduced and this causes a difference in the amount of pressurized oil supplied to the left and right travelling motors TL and TR. Therefore, the construction equipment will not keep travelling straight but will turn left undesirably. Furthermore, when the desired oil pressure of the work implement, which should be manipulated at the same time as driving the vehicle, is higher than the oil pressure of the travelling motor, such as when the arm-out motion will be carried out, the work implement may not be manipulated at all.
Accordingly, the hydraulic circuit should be equipped with a straight travelling apparatus for keeping the travelling direction of the construction vehicle straight even when a work implement is being manipulated at the same time as travelling. Typical straight travelling apparatus are designed so that the first and second pumps P1 and P2 deliver their pressurized oils for the left and right travelling motors T1 and TR respectively when no work implement is being manipulated while the construction vehicle is travelling. However, when even one work implement is being manipulated while the construction vehicle is travelling, the apparatus makes the pressurized oil of either the first or second pumps P1 and P2 be equally supplied to the travelling motors TL and TR and makes the pressurized oil of the other pump P1 or P2 be supplied to an actuator for the work implement to be manipulated.
However, the above straight travelling apparatus has a problem. That is, the travelling velocity of the construction vehicle will be abruptly reduced since the pressurized oil of either the first or second pumps P1 and P2 is supplied to the travelling motors TL and TR when even one work implement is being manipulated while the construction vehicle is travelling.
Another problem of the typical apparatus is resided in that the above oil supply operation will be carried out even when the work implement to be manipulated during travel of the construction vehicle requires a minimum oil pressure. This deteriorates off distribution efficiency of the hydraulic circuit of the equipment.